The properties of closed or almost-closed surfaces with beta-emitting walls will be studied by means of powerful Monte Carlo transport models (some of them devised by the investigators.) Possible use of the enclosures in delivering really-uniform surface does to targets that are either within the enclosures or that are externally apposed to windows in the enclosure walls, will be assessed in light of the calculational results. Simple geometries only, e.g., spherical enclosures (radius 5 to 50 cm) containing smaller spheres as targets, will be studied in Phase I. The influence of (1) source cut-off energy (2) source and target radii (3) target off- centering, on surface dose will be calculated as a guide in devising geometries in which surface doses that are independent, or nearly so, of source/target size and shape are obtained. Such independence would permit design of efficient, convenient, inexpensive and safe irradiators for radiobiological, medical and industrial purposes, including uniform irradiation of unbolused shallow cell cultures, total-or partial-skin irradiation of animal and human subjects with little or no photon contamination, and energy-efficient sterilization or other processing. General guidelines for irradiator design and construction, which in Phase II might include use of nonuniform source distributions, would result from Phase I.